Effect of ammonium on the hydraulic conductivity of geosynthetic clay liners

Abstract

Hydraulic conductivity and swell index tests were conducted on a conventional geosynthetic clay liner (GCL) containing sodium-bentonite (Na-B) using 5, 50, 100, 500, and 1000 mM ammonium acetate (NH4OAc) solutions to investigate how NH4+ accumulation in leachates in bioreactor and recirculation landfills may affect GCLs. Control tests were conducted with deionized (DI) water. Swell index of the Na-B was 27.7 mL/2 g in 5 mM NH4+ solution and decreased to 5.0 mL/2 g in 1000 mM NH4+ solution, whereas the swell index of Na-B in DI water was 28.0 mL/2 g. Hydraulic conductivity of the Na-B GCL to 5, 50, and 100 mM NH4+ was low, ranging from 1.6–5.9 × 10−11 m/s, which is comparable to the hydraulic conductivity to DI water (2.1 × 10−11 m/s). Hydraulic conductivities of the Na-B GCL permeated with 500 and 1000 mM NH4+ solutions were much higher (e.g., 1.6–5.2 × 10−6 m/s) due to suppression of osmotic swelling. NH4+ replaced native Na+, K+, Ca2+, and Mg2+ in the exchange complex of the Na-B during permeation with all NH4+ solutions, with the NH4+ fraction in the exchange complex increasing from 0.24 to 0.83 as the NH4+ concentration increased from 5 to 1000 mM. A Na-B GCL specimen permeated with 1000 mM NH4+ solution to chemical equilibrium was subsequently permeated with DI water. Permeation with the NH4+ converted the Na-B to “NH4-bentonite” with more than 80% of the exchange complex occupied by NH4+. Hydraulic conductivity of this GCL specimen decreased from 5.9 × 10−6 m/s to 2.9 × 10−11 m/s during permeation with DI water, indicating that “NH4-bentonite” can swell and have low hydraulic conductivity, and that the impact of more concentrated NH4+ solutions on swelling and hydraulic conductivity is reversible.